Doped polycrystalline silicon (pcSi) films have been used in various silicon (Si) electronic devices as interlayers between active device layers and metal contacts, and contribute to high gain in bipolar junction transistors by lowering the base current and the emitter resistance. In these structures, the presence of an intermediate, tunneling thickness (e.g., <10 nm), silicon oxide (SiOx) layer between the pcSi and the single crystal silicon wafer provides wafer surface passivation without the degradation of transport. Shallow emitters are formed by diffusing dopants from the pcSi through the SiOx into the wafer via post-deposition anneals. This may be carefully optimized to avoid possible detrimental side effects, such as oxide break-up, secondary phase formation, and blistering. Additionally, dopants tend to segregate along grain boundaries and pile-up at the pcSi/SiOx interface. This has been shown to increase passivation by lowering carrier mobility along grain boundaries in the pcSi, and to chemically bond to dangling bonds in the SiOx.
When stacks made of doped pcSi formed on SiOx are used in solar cells, the SiOx interlayer may provide surface passivation of the underlying wafer. Furthermore, current from the doped pcSi layer may pass through the SiOx layer (e.g., via leakage/tunneling), thereby enabling low contact resistance. Since the heavily doped pcSi is separated from the wafer by the SiOx, there may be no need for additional surface passivation (as in related art Si cells, e.g. by silicon nitride (SiNx)), and the metal contacts can be applied directly to the pcSi. Therefore, pcSi-on-SiOx contacts to Si wafers may provide a way to mitigate metallization degradation while enabling selective carrier extraction. This has resulted in very high efficiency cells that are process-temperature tolerant.
Traditionally, pcSi has been deposited using Low Pressure or Atmospheric Pressure Chemical Vapor Deposition (LP-, AP-CVD) at temperatures over 550° C. However, this results in double sided deposition or a wrap-around problem. Single-side approaches offer the flexibility of additive processes, without the need for film removal.